This invention relates generally to backup seals and, in particular, to a passive backup seal for fluid sealing between a shaft and a radially disposed housing through which the shaft extends, the backup seal being inactive during normal operation, but coming into sealing engagement with the shaft after a failure of a main seal or seals associated with the shaft.
Fluid handling machines, such as pumps or hydraulic piston devices, often include a rotating or reciprocating shaft mounted within a housing. To reduce leakage of pressurized fluid through the space between the housing and the shaft, a seal or a plurality of seals may be used. Conventional seals used for such applications include lapped face mechanical seals, labyrinth seals, packed glands, O-rings, and other similar well-known devices.
In high pressure and/or high temperature applications or where the fluid being handled is toxic or otherwise contaminated, a backup seal is often employed to contain the handled fluid should the main seal or seals fail. For example, in a high pressure pump incorporating a plurality of mechanical seals to break down the pressure from the inside of the pump to the atmosphere, an additional mechanical seal may be used to serve as a backup should one or more of the other seals fail. This additional mechanical seal, however, has certain disadvantages. In particular, the additional mechanical seal requires lubrication and is subject to wear during normal operation of the machine. Alternatively, an actuated closure mechanism may be used wherein a sensor is employed to detect whether a failure condition has occurred, and, upon detection, the sealing mechanism is actuated. In this instance, the sealing mechanism is not used during normal operation of the machine, but is actuated only after a failure. Such a system, however, requires added instrumentation to detect the failure mode and to actuate the sealing mechanism, and, itself, may fail.
A passive device presently used as a backup seal is a segmented, carbon graphite ring disposed around the shaft. The ring is mounted so as to have a relatively close clearance with respect to the shaft. Upon failure of the main seal, the pressurized fluid applies a radial force against the radially outer diameter of the ring. Because carbon graphite has a relatively low modulus of elasticity, it will deform and shrink upon application of a sufficient radial force to close upon the shaft. While the carbon graphite ring described above functions generally satisfactorily, it nevertheless has its own disadvantages. In particular, due to the relatively close clearance that must be maintained between the ring and the shaft to insure proper functioning upon failure of the main seal, the ring tends to wear against the shaft during normal operation of the machine and may, therefore, not seal completely when actuated because of the wear.
It should therefore be appreciated that there is still a need for a backup seal that is completely passive during normal operation of the fluid handling device, but which will immediately and effectively seal the shaft upon failure of the main seal system. The present invention satisfies this need.